1- Faculty of Engineering of Bu-Ali Sina University & School of civil Engineering, University of Tehran , vahidouhadi@yahoo.ca
2- Faculty of Eng., Bu-Ali Sina University
3- Faculty of Engineering, Bu-Ali Sina University
4- Bu=Ali Sina University
2- Faculty of Eng., Bu-Ali Sina University
3- Faculty of Engineering, Bu-Ali Sina University
4- Bu=Ali Sina University
Abstract: (330 Views)
The solidification/stabilization of bentonite and heavy metals is among the conventional methods in geo-environmental projects. Among the various methods used for the solidification/stabilization process, cement-based systems are widely used due to their relatively low cost, availability, and environmental compatibility. Cement-based solidification/stabilization technology is an attractive option for managing heavy metal contaminants and facilitating final transportation and containment, thereby reducing contaminant emissions to the environment. The efficiency of solidification/stabilization technology can be improved through certain modifications. The objective of this paper is to determine the effect of substituting calcium carbonate on improving the solidification/stabilization process of bentonite and heavy metals towards reducing cement consumption.
To achieve this goal, samples of bentonite containing 100 cmol/kg-soil concentration of lead nitrate with different compositions of cement and calcium carbonate were solidified/stabilized. To determine the appropriate concentration of added contamination to the soil, a series of tests for heavy metal retention using the soil suspension equilibrium method, based on EPA standards, has been conducted. These tests were performed on bentonite suspensions at heavy metal lead concentrations ranging from 0 to 250 cmol/kg-soil. In EN197 standard, two types of Portland limestone cement are introduced with the names II/A-L and II/B-L, containing 6 to 20 percent and 21 to 35 percent calcium carbonate, respectively (EN197-1, 2000). Based on this, in the present study, up to 25 percent by weight of calcium carbonate is used as a substitute for ordinary Portland cement, and the combination of cement and calcium carbonate as a binder is used. The mechanism of contaminant retention was evaluated through XRD, TCLP, pH, and UCS tests. In this study, the amount of immobility of heavy metals after stabilization and solidification process using the Toxicity Characteristic Leaching Procedure (TCLP) based on EPA-1311 method has been evaluated. In the first stage of the aforementioned experiment, the solidified/stabilized contaminated sample was adjusted to pH 8.2 with a 0.1 molar hydrochloric acid solution and prepared as a suspension with an S:W ratio (solid:water) of 1:20. All suspensions were continuously shaken for 18 hours using a mechanical shaker, and after measuring the pH of the samples and centrifuging them, the liquid phase was separated and the contaminant concentration was measured using a GBC932 AB Plus atomic absorption spectrophotometer. Unconfined compressive strength (UCS) can be used as a criterion for assessing hydration reaction progress. In this study, samples were subjected to curing for 7 and 28 days in a closed system and placed in a humid chamber at 23 degrees Celsius and 95% humidity according to ASTM D1633-17 standard, with a uniform density of 1.85 g/cm³ for testing unconfined compressive strength. Furthermore, X-ray diffraction (XRD) analysis was utilized to investigate the microstructure of the samples and the progress of the cement hydration process and its interaction with contaminated clay minerals.
According to the results of this study, replacing 15% calcium carbonate instead of cement preserves the necessary conditions for the establishment of stabilization and solidification mechanisms. For instance, for a sample containing 1.4% optimum moisture, the desorption amount of lead ions in the TCLP test is equal to 2 milligrams per liter, and the uniaxial resistance of the sample is equal to 1.45 MPa, meeting both EPA standards. In fact, the achieved results indicate that substituting up to 15% calcium carbonate instead of ordinary Portland cement not only reduces cement consumption but also improves the contaminant retention capability in the cementitious solidification process. The reason for the improvement in these conditions is attributed to the simultaneous role of calcium carbonate filling and nucleation alongside the increase in the range of carbonate compound sedimentation.
To achieve this goal, samples of bentonite containing 100 cmol/kg-soil concentration of lead nitrate with different compositions of cement and calcium carbonate were solidified/stabilized. To determine the appropriate concentration of added contamination to the soil, a series of tests for heavy metal retention using the soil suspension equilibrium method, based on EPA standards, has been conducted. These tests were performed on bentonite suspensions at heavy metal lead concentrations ranging from 0 to 250 cmol/kg-soil. In EN197 standard, two types of Portland limestone cement are introduced with the names II/A-L and II/B-L, containing 6 to 20 percent and 21 to 35 percent calcium carbonate, respectively (EN197-1, 2000). Based on this, in the present study, up to 25 percent by weight of calcium carbonate is used as a substitute for ordinary Portland cement, and the combination of cement and calcium carbonate as a binder is used. The mechanism of contaminant retention was evaluated through XRD, TCLP, pH, and UCS tests. In this study, the amount of immobility of heavy metals after stabilization and solidification process using the Toxicity Characteristic Leaching Procedure (TCLP) based on EPA-1311 method has been evaluated. In the first stage of the aforementioned experiment, the solidified/stabilized contaminated sample was adjusted to pH 8.2 with a 0.1 molar hydrochloric acid solution and prepared as a suspension with an S:W ratio (solid:water) of 1:20. All suspensions were continuously shaken for 18 hours using a mechanical shaker, and after measuring the pH of the samples and centrifuging them, the liquid phase was separated and the contaminant concentration was measured using a GBC932 AB Plus atomic absorption spectrophotometer. Unconfined compressive strength (UCS) can be used as a criterion for assessing hydration reaction progress. In this study, samples were subjected to curing for 7 and 28 days in a closed system and placed in a humid chamber at 23 degrees Celsius and 95% humidity according to ASTM D1633-17 standard, with a uniform density of 1.85 g/cm³ for testing unconfined compressive strength. Furthermore, X-ray diffraction (XRD) analysis was utilized to investigate the microstructure of the samples and the progress of the cement hydration process and its interaction with contaminated clay minerals.
According to the results of this study, replacing 15% calcium carbonate instead of cement preserves the necessary conditions for the establishment of stabilization and solidification mechanisms. For instance, for a sample containing 1.4% optimum moisture, the desorption amount of lead ions in the TCLP test is equal to 2 milligrams per liter, and the uniaxial resistance of the sample is equal to 1.45 MPa, meeting both EPA standards. In fact, the achieved results indicate that substituting up to 15% calcium carbonate instead of ordinary Portland cement not only reduces cement consumption but also improves the contaminant retention capability in the cementitious solidification process. The reason for the improvement in these conditions is attributed to the simultaneous role of calcium carbonate filling and nucleation alongside the increase in the range of carbonate compound sedimentation.
Keywords: Calcium Carbonate, Stabilization/Solidification, Cement, Contaminant Retention, TCLP, Bentonite.
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